Children with autism display a variety of unusual and repetitive activities, interests, and behaviors (RAIBS) that are viewed as "essential" features of the disorder, such as preoccupations, rituals, compulsions, stereotypes, and strong reactions to minor environmental changes. Many children with autism also display severe destructive behaviors that are associated features of autism, such as aggression, pica, property destruction, and self-injurious behavior. Both the essential and associated features of autism markedly interfere with the child's development and cause intolerable hardship to family life. Functional analysis methods have been used to better understand and treat aberrant behavior because these methods permit precise identification of (a) the environmental contexts (i.e., antecedent conditions) in which aberrant behavior is likely and unlikely to occur, (b) the consequences that reinforce or maintain the behavior, and (c) effective treatments. We have begun to apply the functional analysis method to these essential and associated symptoms of autism. Our preliminary results suggest three operant patterns for these symptoms. In one pattern, the essential and associated symptoms have common functions (i.e., both maintained by sensory reinforcement automatically produced by the responses). In the second pattern they have independent functions (e.g., one maintained by sensory reinforcement, the other by escape from demands). In the third pattern they have interrelated functions, in which access to an essential symptom of autism (e.g., stereotypy) functions as reinforcement for the associated symptom (e.g., aggression). Each pattern leads to a distinct behavioral intervention that is differentially effective in reducing both the essential and associated symptoms of autism. The current project is designed to further examine the common, separate, and interrelated functions of the essential and associated symptoms of autism. We hypothesize that categorizing and analyzing these essential and associated symptoms in terms of both their structural and functional properties will enhance our understanding of autism and lead to more effective treatment.